However, ST:NEM then indicates that the particular region of the RNZ shown in the "BoT" map remains unaltered in a late 24th century floor mosaic, for the relevant part.

It's a mosaic, not a strategic chart. It was probably installed on the senate floor a hundred years earlier when Romulus was finally unified enough to try to break out of isolation again.

All respect to preservation of ancient art and whatnot - but would it be plausible that the Romulan Senate would continue to feature artwork depicting a political situation that is significantly less advantageous to Romulus than the true, current one?

There's something to be said for nostalgia, especially when you consider that most of those senators were probably youngsters when Balance of Terror went down. It wouldn't be ancient to them, it would be a reminder of how far they've come in their own lifetimes.

flying around it (which they seem to do in "Face of the Enemy")

It seems to me the Romulans have a way of flying around it in episodes like "Tin Man", "The Next Phase" and "Paradise" and perhaps "Eye of the Needle" as well, since Romulan presence outside the RNZ goes unexplained there. But in "Face of the Enemy", Romulan penetration out of confinement is actually discussed, and Troi claims Tal'Shiar codes will make it possible to avoid detection within UFP territory. This is never put to a test, because the Romulans already meet the E-D before launching deeper into UFP space - so the indications are that the Romulans had previously successfully escaped from the RNZ into UFP space where the E-D now justifiably dwells, and that this was all due to nothing but good cloaking discipline.

Actually, "Face of the Enemy" mentions the Federation's border protection grids which are designed to detect cloaked ships. It doesn't seem they were worried about the neutral zone so much as the security perimeter around the Federation itself. That suggests to me that while the neutral zone is a POLITICAL buffer before the two powers, it's not actually an obstacle to the Romulan fleet who can easily circumvent it if they need to. The TACTICAL obstacle in that or any other case would be the Federation's (apparently automated) defense grid that can detect a cloaked ship before it gets in striking range. I would imagine those missiles from the Mars Defense Perimeter would actually be pretty effective against a Romulan Warbird.

True, but then he wouldn't be standing there in the President's office giving legal advice while the Klingon ambassador barking about the extradition of Kirk. That would earn him at least one pointed "What the fuck are YOU doing here?" remark.

Hmm... Generally, visual communications don't reveal much of the surroundings.

They do when the ambassador is physically standing there IN the office, openly talking to the guy, asking him things like "What is the position of the Romulan government, ambassador Nonclus?" and "You can't seriously believe that James Kirk assassinated the chancellor of the high council?!"

What's significant is that nobody thinks it's odd for Nonclus to be in that office or to be present for that confidential briefing. Relations between the Federation and Romulus were never THAT good.

There's nothing odd as such about him being present when the Klingons call the President, though.

That's mainly because the Klingon holocom never shows anything BUT the president.

I'm talking about the pissed off ambassador who physically goes there to insist that Kirk and McCoy must stand trial in a Klingon court.

Interestingly, ST:INS would have us believe that Romulans went from "thugs" to an "empire" around this time; since nothing much else is known to have changed about them, the "empireness" might come simply from them engaging in open diplomacy with their enemies for the first time.

It could, sure, but even you can see that this would ALSO be consistent with their finally expanding far beyond their own solar system and becoming a major galactic power.

Also interestingly, Kor would in "Blood Oath" boast of a victory against the Romulans in a timeframe preceding this movie; if there ever was an alliance between the Klingons and the Romulans, it would seem to have gone sour in the mid-2270s, making it plausible for Nanclus to bring messages of goodwill to the UFP in ST6.

I was remembering "Blood Oath" when I made the above posts. That's why I had believed that the Romulans had stopped cooperating with the Klingons and went from being a semi-willing subject of the empire to an occupied (and resistant) territory. Klingon governance is apparently harsh enough that rebellions against the empire are not uncommon, even among Klingons.

Final intrigue comes from the fact that Nanclus and Sarek share a sash color and a seating sector at Khitomer. Also, the background colors of flags are telling: UFP flag corresponds to the blue UFP sash, Klingon flag (the Nazi-alluding red-white-black one is seen here for the first time, I think) corresponds to their red sash, while both the Romulan Bird of Prey and the Vulcan IDIC are seen on yellow background in flags and banners. Is there something going on with a political block involving all Vulcanoids, despite Vulcan's supposed continuing membership in the UFP?

As I said, a government in exile. I'd be willing to bet the Romulan Emperor sought asylum on Vulcan after Klingon control of Romulus turned into a war of occupation. If nothing else, that might explain how the Vulcans managed to repair a captured bird of prey that they had otherwise never seen before: maybe those weren't really vulcans?